1. Field of the Invention
This invention relates to the preparation of semiconductor alloys of the formula Hg (1.sub.-x) Cd.sub.x Te.sub.1, where "x" is in the range from 0.14 to 0.40 for infrared applications, generically referred to as "HgCdTe".
2. Description of the Prior Art
Alloys composed of mercury, cadmium, and tellurium and having the general formulation set forth hereinabove have been reported in the prior art, particularly in regard to their semiconductor properties and their use in conjunction with infrared detectors and the like. These materials often include mobile residual impurities of undesirable types which can act as dopants and also can limit the performance or contribute to a decrease of normal life expectancy of any device produced from the alloy. The most common manifestation of impurity found in HgCdTe alloys is an acceptor which commonly exists in the mid 10.sub.15 /cm.sup.3 range and a residual donor impurity commonly in the range of 1-10.times.10.sup.14 /cm.sup.3 range. During quenching the alloys from high temperatures, precipitates are formed everywhere in the alloy. The subsequent low temperature post-anneal step in-diffuses Hg which annihilates the tellurium precipitates near the alloy surface, leaving a core region which is P-type due to being saturated with excess Te. If a core is left, this is because the post-anneal was not continued long enough since annihilation of the excess tellurium takes place from the surface toward the central region during post annealing. The residual acceptor impurities may be gettered into the core retgion of the alloy in a known manner, the core region normally being a region of excess or second phase tellurium which remains after the post-annealing step in a saturated mercury ambient atmosphere as noted above. Presence of the core during the in-diffusion of Hg results in a net migration of acceptor impurities into the core region. The result is an N-type skin on the surface of the alloy with a typical carrier concentration in the surface retion in the mid 10.sup.14 /cm.sup.3 region. When the core is annihilated by prolonged post-annealing or the like, as is discussed in greater detail in copending application Ser. No. 564,953, filed Dec. 23, 1983 (TI-9916), the second phase and excess tellurium are initially removed and then the impurities will tend to migrate homogeneously throughout the alloy by solid state diffusion. This does not present a problem when the impurities are desirable and required as a dopant in the existing concentrations. However, these impurities, normally being in the form of acceptor impurities, are released from the core region, and can result in a conversion of the surface of the alloy back to P-type in the mid 10.sup.15 /cm.sup.3 range. It is, therefore, clear from the above discussion that it is desirable and often necessary that the unwanted impurities can be removed from the HgCdTe alloy.
In current practice, to the knowledge of the applicants herein, there are no conscious attempts being made to getter impurities in HgCdTe and physically remove them from the system. Some attempts have been made to remove the impurities by mechanical backside damage; however, current theoretical experimental understanding suggests that this approach will be unsuccesful.